The technique disclosed in the present application relates to switching regulators for supplying DC (direct current) voltage to various electronic devices, and current sensing circuits used for such switching regulators.
In recent years, switching regulators have been used as power supplies for a variety of electronic devices because of their high-efficiency power conversion. A buck converter, which is an example of such switching regulators, includes a switching transistor for chopping at a high frequency a DC input voltage of 3-4 V supplied by, for example, a lithium ion battery; an inductor for smoothing a pulsed output from the switching transistor; an output capacitor; and a control circuit, thereby supplying a DC power supply voltage of 1-2 V to a load.
The control circuit monitors the output voltage supplied from the output capacitor to the load, thereby controlling an on/off time of the switching transistor so that the output voltage is regulated. Examples of methods for controlling the switching regulators include current mode control methods, which have frequently been used, where the output voltage is regulated by controlling the switching transistor by sensing the current flowing through the inductor.
FIG. 8 is a circuit diagram showing an example configuration of a conventional switching regulator. The switching regulator of FIG. 8, which is described in U.S. Pat. No. 6,377,034, includes an inductor current sense circuit 900. FIG. 9 is a graph showing signal waveforms in the switching regulator of FIG. 8. In FIG. 8, a DC voltage Vin is input to an input node T1. A drive signal for switching a transistor 2 is applied to the gate of the transistor 2. The drain of a rectifying transistor 4 is connected to the drain of the transistor 2. A comparator 5 compares the drain voltage of the transistor 4 with ground potential, and drives the transistor 4 by a signal indicative of the comparison result.
One end of an inductor 6 is connected to a node connecting the transistor 2 to the transistor 4. An output capacitor 7 is connected to an output node T2, which is another end of the inductor 6. An output voltage Vout is supplied from the output node T2 to the load. A current flowing through the inductor 6 is (IL1+IL2), where a current flowing through the transistor 2 is denoted by IL1, and a current flowing through the transistor 4 is denoted by IL2.
The source and gate of a sense transistor 912 are connected to the source and gate of the transistor 2, respectively. The size of the sense transistor 912 is assumed to be 1/N times that of the transistor 2. Since the drain-source impedance of a transistor 922 is adjusted by a differential amplifier 914 so that the drain voltage of the transistor 2 is equal to the drain voltage of the transistor 912, a current of IL1/N flows through the transistors 912 and 922. The transistor 922 and a transistor 928 form a current mirror, and thus, the current of IL1/N flows through the transistor 928.
The drain and gate of a transistor 942 are connected to the drain and gate of the transistor 4, respectively. The size of the sense transistor 942 is assumed to be 1/N times that of the transistor 4. Since the drain-source impedance of a transistor 958 is adjusted by a differential amplifier 944 so that the source voltage of the transistor 942 is equal to ground potential, a current of IL2/N flows through the transistors 942 and 958. A current flowing into a node TC will be (IL1+IL2)/N, and thus, the inductor current (IL1+IL2) can be sensed.
However, the current sense circuit 900 of FIG. 8 poses a problem that the current cannot be sensed during a dead time, etc. when both of the transistors 2 and 4 are turned off. As shown in FIG. 9, for example, the current IL flowing through the inductor is continuous, whereas the current (IL1+IL2)/N obtained by the current sense circuit 900 is discontinuous. In particular, average current mode control schemes, which control an average value of the inductor current, present a problem that it is difficult to control a switching regulator according to such a discontinuous signal.